1. Field of the Invention
The present invention generally relates to a plasma addressed display (PAD) device having an electro-optical part. The invention more particularly relates to a plasma addressed liquid crystal (PALC) display device comprising plasma channels for addressing a liquid crystal layer used as the electro-optical part, the channels being filled with a mixture of main ionizable gas and its dopant gas.
2. Prior Art
PALC displays of such a type are well known from European patent publications Nos. EP 0 816 898 A2 and EP 0 833 363 A2 and so on.
The former describes a PALC display device with an integrated source of reactive gas. Said integrated source comprises a body of a storage material which is capable of storing a dopant gas to be mixed into a primary ionizable gas in plasma channels and reversibly absorbs and releases the dopant gas. In this display device, use of the source allows an equilibrium to be established between concentration of the dopant gas in the body of the storage material and partial pressure of the dopant gas in the plasma channels. This enables to keep the partial pressure of the dopant gas constant in the plasma channels. Hydrogen (H) is typically used for said dopant gas, and helium (He) is typically used for said main ionizable gas.
The latter also describes a PALC display panel comprising a source of dopant gas similar to the source mentioned above, but further comprising an electrical resistance heater to improve performance of the source. The heater is used to heat the storage material of the source and elevate the partial pressure of hydrogen in the panel so that the equilibrium is established between the concentration of hydrogen in the storage material and the partial pressure of hydrogen in the panel. In this manner, it is possible to achieve and maintain the desired partial pressure of hydrogen in the panel even though the storage material has initially lower concentration of hydrogen. Altematively, or in addition, use of the heater allows the useful life of the panel to be prolonged. That is because the use of the heater can delay the time at which the partial pressure of hydrogen in the panel falls below a desired value for reason of the limits of the storage material""s natural ability, by virtue of increasing the temperature of the storage material.
However, this prior art of EP 0 833 363 A2 does not suggest how to heat the storage material and in particular does not operate the storage material and heater with precision. Hydrogen in channels is consumed by various largely unknown parameters not only during normal operation of the display but also during off time of the display device, namely during all over the lifetime. It is, therefore, very important to adapt the amount of hydrogen released or absorbed by the storage material to a changing real-operating/environment conditions of the display device. If the amount of hydrogen to be adapted to the conditions were too small, a lower hydrogen partial pressure might be applied to the channels, which may lead to too long decay times of the channels and a deteriorated addressing of the line pixels (line-sequentially addressing operation).
A decay time refers to a conductivity transition period starting from turning off the plasma in a channel, during which the plasma conductivity caused by the plasma returns to, i.e., transits to substantially zero. It can be also said that the decay time depends on a switching speed of the gas filling in the channel from ON-state to complete OFF-state in that channel. The too long decay time must be avoided especially for displays having a large number of scanning lines such as HDTV displays and computer monitors because a period of time assigned for one scanning line is shorter in such types of displays.
It is, therefore, an object of the present invention to provide a plasma addressed display device which can adapt the amount of dopant gas released or absorbed by the storage material to a changing real-operating/environment conditions of the display device in order to control the partial pressure of the dopant gas in the channel.
It is another object of the present invention to provide a plasma addressed display device which can operate by a faster switching gas supplied to plasma channels.
For achieving the above-mentioned objects, a plasma addressed display device according to the present invention comprises:
a channel substrate provided with an array of interconnected longitudinal channel chambers;
a cover sheet extending over the channel chambers and making seals to the channel chambers on a side of an upper face of the chamber, whereby the channel substrate and the cover sheet define sealed channel chambers;
a pair of electrodes located in each sealed channel chamber for selectively ionizing a gas within the channel chamber;
a dopant source having a storage material that reversibly absorbs and releases a dopant gas to be mixed with a primary ionizable gas in the sealed channel chambers; and
a heater for heating the storage material,
which is CHARACTERIZED in that there is provided a feedback loop for a control of partial pressure of the dopant gas within the sealed channel chambers, the feedback loop comprising:
measurement means for measuring a value corresponding to a decay time of an electrical conductivity in at least one of the channel chambers from turning off the plasma in the channel chamber; and
control means for controlling a temperature of the storage material through the heater on the basis of the measured value so that the decay time is within a predetermined range.
According to the plasma addressed display device having the above-mentioned structure, it is possible to adjust and optimize an amount of the dopant gas released from the storage material in accordance with changes of the real decay time of the channel conductivity by controlling the temperature of the storage material, like a servo system. That is, regardless any conditions of the display a feedback control can be realized, which exactly maintains a desired partial pressure of the dopant gas in the channel chambers. The device according to the invention, therefore, offers an advantage in that the display operation, especially the line-sequentially scanning operation can be maintained appropriately over the lifetime. It further offers an advantage in that a shorter time is required for addressing each line, and the device becomes easy to be adapted for displays in which an assigned scanning time per line is shorter, e.g., HDTV displays and computer monitors since the channel chamber can be filled with the gas faster in switching.
In the display device, the measurement means may perform its measurement by sensing the amount of space charges remained in at least one of the channel chambers a predetermined time after plasma discharge has been completed in the channel chamber.
The measurement means may comprise: a switching circuit for changing a current path of one of the electrodes between first and second outputs thereof; and a capacitor connected to the first output, the second output being coupled to a reference potential for plasma discharge in the chambers, the switching circuit changing the current path from the second output into the first output in response to completion of a plasma-ignition pulse which is applied to the other of the electrodes.
The measurement means may further comprise means for measuring a voltage across the capacitor and for generating a decay time signal showing the decay time length in accordance with the measured voltage.
Furthermore, the control means may comprises: a comparator for comparing a value of the decay time signal with a predetermined reference value to generate a control signal according to a result of the comparison; and a driver for generating a drive signal in accordance with the control signal to supply the heater.
The above-mentioned aspects and the other aspects according to the present invention will be more specifically described with reference to the accompanying drawings.